Steam generating unit



July 30, 1957 M H, KUHNER 2,800,889

STEAM GENERATING UNIT Filed Sept. 20, 1952 2' Sheets-Sheet 1 Fig.1

INVENTOR MAX fl. KUHNER ATTORNEY M. H. KUHNER STEAM GENERATING UNIT 2 Sheets-Sheet 2 Filed Sept. 20, 1952 lmENroR Max A. KUHNER ATTORNEY aten 2,800,889 Patented July 30, 1957 I 2,800,889 STEAM GENERATING UNIT Max H. Kuhner, Worcester, Mass, assignor to Riley Stoker Corporation, Worcester, Mass., a corporation of Massachusetts Application September 20, 1952, Serial No. 310,657

2 Claims. (Cl. 122-480) This invention relates to steam generating units and more particularly to the arrangement of vapor-carrying tubes in an apparatusfor producing steam.

When considering the design of steam-generating equipment, particularly for large installations, it is neces sary to assume that the quality of available fuels will continue to decline as time goes on. The better grades of coal are needed for the metallurgical industry and for plants where the design of furnaces and fuel-burning equipment necessitates the use of premium coals. Furthermore, difficulties such as slagging and fouling of boiler-heating surfaces can become just as serious with oil as with low-grade coal. This is because improved refinery processes have brought about a greater percentage of impurities in the residual fuel oils which are used in power plants. Also, although natural gas is an ideal fuel, at present a dependable supply exists only around oil and gas fields. Now, in burning low-grade coals such as strip mine coal, which contains over 15% ash which fuses between 1900 and 2000 degrees F., it is necessary to provide enough cooling within the furnace to prevent ash fusion, which means that the heat-release rate must be kept below 75,000 B. t. u. per hour per square foot of furnace envelope. In this way the furnace exit gases are at a temperature of around 1900 degrees F. and slagging does not take place. However, when a superheater and a reheater are used, the amount of heat left in 1900 degree Fpcombustion gas may be too low to heat these components by convection alone. Furthermore, in the past it was not thought possible to provide a reheater section which was subject to substantial radiant heating because it was believed that the tubes in such section would fail. A purely convective reheater combines with the unavoidable rise in reheater temperature with load to give extreme rise in temperature wit-h higher load. This means that convective reheaters are extremely sensitive to changing conditions and are very unstable. The present invention obviates these difficulties in a novel manner. It is therefore an outstanding object of the present invention to provide an arrangement of superheater and reheater tubes in a steam generating unit which will permit the maintenance of low combustion gas temperatures while maintaining high superheat and reheat temperatures.

Another object of this invention is the provision of a steam generating unit which may use fuels having a low temperature of fusion without objectionable slagging or fouling of boiler surfaces.

It is a still further object of the invention to provide an arrangement of superheater and reheater surfaces which will provide for the superheating and reheating of steam in a manner which is substantially independent of load.

Furthermore, it is an object of the instant invention to provide an arrangement of superheater and reheater surfaces wherein a portion thereof are exposed to the radiant heat of a furnace in a manner not considered possible heretofore.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of my invention.

Figure 1 is a longitudinal sectional view of a steam generating unit embodying the present invention,

Figure 2 is a longitudinal sectional view of a portion of the unit shown in Figure 1 somewhat enlarged, and

Figure 3 is a horizontal sectional view taken on the line IIIIII of Figure 2.

Referring first to Figure 1, wherein is best shown the general aspects of the invention, the steam generating unit, designated generally by the reference numeral 10, is shown as composed of a furnace 11 and a boiler 12. The furnace is of the type designed for burning pulverized, low-grade coal and is provided with water walls 13 culminating at the lower portion in a hopper 14. Mounted on the forward wall of the furnace are a number of burners 15 arranged in several rows, there being several burners in each row. The boiler 12 consists of a steamand-water drum 16 from which extend downcomer tubes 17 and into which extend steam-release tubes 18; for the purpose of clarifying the showing, only a portion of each tube is shown. Steam tubes 19 leave the top of the steam-and-water drum and are attached to the lower portion of a first-stage or low-temperature superheater 20 which is located in the back pass of the boiler. The upper portion of the first-stage superheater is connected to a header 21, which in turn is connected to the forward portion of a second-stage or high-temperature superheater 22 which is located in the forward part of the horizontal pass of the boiler; the rearward portion of the second-stage superheater is connected by long vertical tubes 23 to the superheater outlet 24. A reheater inlet 25 is connected by long, generally vertical tubes 26 to a header 27 which in turn is connected to the rearward portion of a reheater 28 located behind the second-stage superheater in the horizontal gas pass. The forward portion of the reheater is connected by vertical tubes 29 extending behind the forward wall of the furnace to a reheater outlet 30. The entire setting is suspended from a framework 31 in the usual manner.

The back wall of the furnace is provided with an extension 32 at its upper edge consisting of a forwardly inclined portion 33 merging into a vertical portion 34 which extends upwardly a short distance between the second-stage superheater 22 and the reheater 28. A protuberance 35 formed of refractory material is provided on the rear surface of the back wall of the furnace and has a thick vertical section 36 and a downwardly and rearwardly inclined lower section 37. A vertical baffle 38 extends transversely of the furnace intermediate of the back wall of the furnace and the outer, rearward wall 39 of the entire unit. Underneath the baffle 38 is situated another bafile 40 having an upper portion 41 which is on the same level and parallel with the inclined surface of the lower section 37 of the protuberance 35; the portion 41 merges into a short, vertical portion 42 which extends downwardly therefrom. A series of adjustable dampers 43 extend transversely across the back pass of the furnace and overlie a hopper 44 which extends downwardly therefrom. The inlet header 45 of an economizer 46 is situated at the lower end of the bafile 40, while the outlet header 47 extends along the top edge thereof. The economizer itself extends across the back pass of the boiler with a considerable portion residing behind the baflie 40 and a smaller portion extending forwardly of the baffle; the portion of the econheater can see the furnace to, a certain extent.

leaving the rearward portion of the second-stage superomizer adjacent the outlet header resides only in the 7 pass between the bafile 40 and the back wall of the furnace.

The particular arrangement of the tubes in the super-, heater and the reheater are shown clearly in Figure 2' where the other structures. in the unit are omitted. It is particularly to be noted in this .view that the vertical tubes 26 leading from the reheater inlet 25 to the header 27 consist of vertical sections 48 which extend along the'forward surface of the backwall of the furnace, up-

. Wardly and outwardly inclined sections 49 which, lie

along the forward surface of the inclined portion '33 of the 'extension 32 for a short distance and extend therebeyondto underlie'the second-stage superheater 22 before extending vertically upwardly to the roof of the furnace.

The second-stage superhcater22 and the reheater 28 are of the dependent type having vertical loops which lie' in planes extending longitudinally of the furnace. It is to a be noted, also, that the tubes 23 which-extend from the rearward portion of the second-stage superheater 22 to the superheater outlet 24 follow the rearward surface of the-back wall of the furnace rather closely, while the tubes 29 which extend from the forward portionof the reheater 23 to the reheater outlet 30 Weave back and forth between the back wall and the bafiles' 33 and 40,

7 each tube remaining in a longitudinal vertical plane, of

course. Both tubes 29 and 23, however, closely overlie theinclincd surface of the lower section 37 of the protuberance 35. 'Another feature of note is the manner in: which the tubes19 leading from the steam-and-water drum 16 t the lower portion of the first-stage superheater 20 extend along the surfaces of the baflie 38 and the rear gases. The gases pass horizontally over the top of the extension 32 and then downwardly in the back pass of the boiler between the rear wall 39 and the bafiles 38 and 40 and betweenthe said bafies and the back wall of the furnace. The division of the gases between the two.

parts of the back pass is determined by the setting of 7 the dampers 43. The gases then pass into the hopper 44.

and eventually into the stack, not shown. Steam which has been released in the steam-and-water drum I16 passes into the tubes 19 and flows downwardly therethrough to the lower'part. of the first-stage superheater 20. While it is in the first-stage'superheater, the steam movement is in counterfiow to the flow of gases and, thus, the coolest steam is heated by the coolest gas and the hottest steam is heated by the hottest gas. The steam from the first-stage superheater is collected'in a header 21, where equipment, not shown in the accompanying drawings, is

situated for regulating-the steam temperature. This equipment may be a contact desuperheater. From the header 21 the steam flows through tubes to the forward portion of the second-stage superheater and flows therethrough in the same direction as the hot products of combustion; in this case, the coolest steam is heated by. the hottest gas, while the hottest steam isheated by the coolest gas. This arrangement assures that the skin temperature at the forward portion of the second-stage superheater does not become excessive. .The second-stage superheater is able to receive a small amount of radiant heat fromthe furnace because'of thefact that the tubes26 of the radiant section of the reheater are somewhat spaced and the super, Upon heater, the steam passes downwardly through the tubes 23'to the superheater outlet 24 and'thence to the turbine,

- of the gases passing downwardly through the pass between the back wall of the furnace and the bafiies 38 and 40.

The steam is eventually returned from the first stage of the turbine and is introduced into the reheater inlet '25 and passes upwardly through the tubes 26. For most of the way up to the header 27 the steam is subjected to radiation from the combustion chamber. It was hereto fore supposed that the use of radiant tubes of this type and arrangement would result in burned out tubes, the theory being that tubes containing steam which were situated in an area of such intense radiant heat would become overheated because of. the inability of steam to carry. away the heat and cool the tube metal. However, by using a proper size of tubing for the construction of the tubes 26, that is to say, in the radiant section of the reheater, the velocity of steam passing therethrough is sufficiently great to maintain the skin temperature within reasonable limits. From. the header 27, the steam passes into the rearward portion of the reheater 28 and flows forwardly in counterfiow to the combustion gases, so that the hottest steam is heated by the hottest gasesand the reheat outlet temperature is obtained fromthe hottest gases. The forward portion of the reheater is connected to the reheat outlet 30 by the tubes 29 the bending of the tubes 29 transversely of the passage between the back of the'furnace' wall and the baffies 38 and 40*causes the steam p'ass'ing downwardly through them to be regulated somewhat'by the gases flowing in the. passage, even though the differential between the steam in the'tube:

and the gas outside the tube is not as great asit'is inthe reheater proper. divide thelgas flow between the two passages into which the back pass of the boiler is separated by the bafiles 38 and 40; the division of the flow in such a manner not only permits adjustment of the feed water temperature and the first-stage superheating, but also the final reheat temperature because of its effect on the steam in the tubes 29. As is well-known, the heat recovery rate in steam which is confined in a tube which is subjected to convective heating increases with load, while it drops with" load when the heating is purely radiant. The use ofthe combination of radiant and convection heating as demonstrated herein results in a heat recovery which is substantially independent of load in the normaloperating range. This is of considerable importance in the case of reheating, because although the inlet temperature to a superheater ,is' always the same, the inlet temperature to a coal-fired steam generating unitdesig'ned for the poorest Midwest coals. The strip-mine coal, used contains over 15% ash, which fuses at 1900 to 2000 degrees F. To

provide enough cooling within the furnace tov prevent ash fusion, the heat-release rate had to be kept below 75,000 B. t. h. per hour per square footrof furnace envelope. In the frontwall sixteen burners were so:

arranged that each flame was free to develop without, interference from any other flame. To further insure complete combustion, the furnace was made very high; This was a 600,000 poundsofsteam per; hour reheat boiler, delivering steam at 1000* i. e., one hundred feet.

degrees F. and 1525 p. s. i. g. and reheating steam from the first section of the turbine from 660 to 1000 degrees F. at 450 p. s. i. g. The combination of superheater and reheater recover about 487 B. t. u. per pound of steam' generated. Now if reheater and superheater surfaces were placed to receive convection heat only, and if no The dampersl43 may be adjusted. to

portion of these surfaces were exposed to furnace radiation, all heat recovered by them would come from combustion gas passing over them. Since there is about 1.35 pounds of combustion gas for each pound of steam generated, the temperature of this gas unit would have to drop about 1360 degrees F. to give off the 487 B. t. u. needed for superheating and reheating. But the temperature of the gas leaving the furnace may be only 1900 degrees F. and enters the superheater and reheater area at no higher than 1850 degrees F. A gas temperature drop of 1360 degrees F. would mean flue gas leaves this area at 490 degrees F. This is not possible because the exit gas temperature would be 120 degrees F. lower than the saturated-steam temperature of 610 degrees F. at 1526 p. s. i. g. For effective heat transfer from combustion gas to steam, the temperature diflerence must be at least 100 degrees F. This means that the exit-gas temperature leaving the lowest temperature section of the superheater and the reheater should be at least 710 degrees F. Because the required gas-temperature drop over the superheater and the reheater surfaces is 1360 degrees F., the temperature of gas entering this area cannot be lower than 2070 degrees F., or about 2120 degrees F. leaving the furnace. Severe slagging trouble would result if the exit-gas temperatures were that high. So, in the example under consideration, the superheater and reheater surfaces are placed according to the invention to give partial radiation recovery. This radiation recovery is at least equivalent to a 220 degree F. gas-temperature drop. The superheater and reheater for combination radiant and convective heat recoveries take advantage of the natural characteristics of both. The heat-recovery rate becomes less with increasing load for a purely radiant superheater or reheater, while it becomes greater with load for a purely convective arrangement. The two natural characteristics are so -combined that radiation and convection are in almost exact balance and it is theoretically possible to hold a given steam temperature throughout a wide range of steam load without any other type of steam temperature control. The tubes of the reheater which act as a screen between the furnace and the superheater are steam cooled. The steam entering the reheater is a maximum of 750 degrees F. and approximately 450 p. s. i. g. The velocity of flow of steam through these tubes is in the order of 142 feet per second and amounts to 300,000 pounds per square foot per hour. This compares to 36,000 pounds per square foot per hour for the steamwater mixture flowing through the water-cooled steaming tubes. The steam temperature rise within the radiant portion of the reheater is a maximum of 140 degrees F. so that the maximum mean temperature of the fluid in the reheat screen tubes is about 790 degrees F., i. e., 720 plus 70. This high rate of mass flow of steam passing through the tubes insures effective cooling. Taken with the fact that the walls of the tubes are thinner, this insures that the outer skin metal temperature of the radiant reheat tubes is not more than 50 degrees F. greater than the fluid temperature, or an average of 840 degrees F.

It can be seen then that the use of the present invention enables one to obtain several important advantages over previously-known constructions. First, it is possible to change the overall characteristic curve of the reheater to approximate closely a flat, straight line; that is to say, the outlet temperature is the same irrespective of the load. Secondly, by the use of the radiant section in the reheater, the tube length of the reheater is cut down, which means that there will be less friction drop through the reheater; this is a very important factor, since the amount of work that can be obtained from the steam in the second stage of the turbine is dependent on the pressure left in the steam when it reaches the turbine after reheat. Thirdly, it is possible to operate the furnace in such a way as to obtain a low gas-exit temperature and still obtain the required temperature rise in the reheater; a low-temperature gas leaving the furnace assures that there will be no slagging and also helps to insure against overheating of the tubes in the radiant section of the reheater.

While in accordance with the provisions of the statutes 1 have illustrated and described herein the best form of my invention now known to me, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by my claims, and that certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. A steam generating unit comprising: a furnace having a forward wall, a back Wall and side walls defining a vertically-elongated combustion chamber, a rearward wall defining with the said back wall the back pass of the unit, a bafiie dividing the back pass into a forward portion and a rearward portion, an extension at the upper portion of the said back wall of the furnace causing the hot gases of combustion to flow horizontally from the top of the combustion chamber and vertically downwardly through the back pass, adjustable dampers at the lower portion of the back pass serving to determine the flow of gases through the forward and rearward portions thereof, a low-temperature superheater residing in the rearward portion of the back pass and so connected that steam flows therethrough in a generally upward direction in counterflow to the gases, a high-temperature superheater residing adjacent the extension of the back wall of the furnace and forwardly thereof and so connected that steam flows therethrough in the same general direction as the flow of gases, a radiant reheater section extending up the back wall of the furnace on the side thereof facing the combustion chamber and extending around the high-temperature superheater to screen it from a portion of the radiation from the combustion chamber, a convection reheater situated adjacent the extension of the back wall of the furnace and slightly rearwardly thereof, the said convection reheater being connected so that steam flows therethrough in counterflow to the gases, a portion of the said convection reheater extending along the length of the forward portion of the back pass and extending back and forth thereacross so that steam residing therein is subject to flow of ga therethrough.

2. A steam generating unit, comprising: a furnace having a forward wall, a back wall, and side walls defining a vertically-elongated combustion chamber, a rearward wall defining with the said back wall the back pass of the unit, a baflle dividing the back pass into a forward portion and rearward portion, an extension at the upper por tion of the said back wall of the furnace causing the hot gases of combustion to flowhorizontally from the top of the combustion chamber and vertically downwardly through the back pass, adjustable dampers at the lower portion of the back pass serving to determine the flow of gases through the forward and rearward portions thereof, a low-temperature superheater residing in the rearward portion of the back pass and so connected that steam flows therethrough in a generally upward direction in counter-flow to the gases, a high-temperature superheater residing adjacent the extension of the back wall of the furnace and forwardly thereof and so connected that steam flows therethrough in the same general direction as the flow of gases, a radiant reheater section extending up the back wall of the furnace on the side thereof facing the combustion chamber and extending around the hightemperature superheater to screen it from a portion of the radiation from the combustion chamber, the tubes in the said radiant reheater section being widely spaced compared with the spacing of the tubes in the high-temperature superheater, a convection reheater situated adjacent the extension of the back wall of the furnace and slightly rearwardly thereof, the said convection reheater being connected so that steam flows 'therethrough in counter-flow to the gases, a portion of the said convection reheater extending along the length of the forward portion of the back pass and extending back and forth'therecross so that steam resulting therein is sub- 9 ject to flow of gas therethrough 7 References Citedin the file of this patent UNIIED STATES PATENTS 1,773,088 Armcost Aug. 19, 1930 Armacost -g., Sept. 3, 1940 Woolley May 115, 1951 Lacerenza -Mar. 25, 1952 Boyer June 3, 1952 Van Brunt Feb. 17, 1953 Blaskowski Aug. 3, 1954 FOREIGN PATENTS V 7 France V Jan. 18; 1950 

